In recent years, a fuel cell has been attracting attention as a clean energy source. One example of the fuel cell is a polymer electrolyte fuel cell. The polymer electrolyte fuel cell (hereinafter referred to as a “PEFC”) includes a membrane-electrode assembly, an anode separator, and a cathode separator. The anode separator and the cathode separator are arranged to sandwich the membrane-electrode assembly and respectively contact an anode and a cathode. The membrane-electrode assembly includes the anode and the cathode (which are called “electrodes”). Each of the anode and the cathode includes a gas diffusion layer and a catalyst layer. The gas diffusion layer includes minute holes which constitute a flow path of a reactant gas. A fuel gas channel is formed on one main surface of the anode separator. An oxidizing gas channel is formed on one main surface of the cathode separator. The fuel gas (hydrogen) having been supplied through the fuel gas channel to the anode is ionized (H+), flows through the gas diffusion layer and catalyst layer of the anode, further flows through the polymer electrolyte membrane via water, and moves to the cathode side. The hydrogen ions having reached the cathode side generate water through the following electric power generating reaction in the catalyst layer of the cathode.
Anode Side: H2→2H++2e−
Cathode Side: (½)O2+2H++2e−→H2O
Total Reaction: H2+(½)O2→H2O
The water (generated water) generated as above flows into the oxidizing gas channel of the cathode separator as steam or liquid. Moreover, a part of the water generated in the cathode side moves to the anode side (so-called “back diffusion”) to flow into the fuel gas channel. The generated water having flowed into the oxidizing gas channel or the fuel gas channel moves to a downstream side with the flow of the oxidizing gas or the fuel gas. On this account, local variations in the amount of moisture in the electrode may become large, and as a result, local variations in the amount of electric power generation may become large.
To solve such problems, a fuel cell is known, in which: first channels through which a gas flows in and second channels through which a gas is discharged are included; the first channel on the anode side and the second channel on the cathode side are opposed to each other so as to sandwich a polymer electrolyte layer; and the second channel on the anode side and the first channel on the cathode side are opposed to each other so as to sandwich the polymer electrolyte layer (see PTL 1, for example). Moreover, a polymer electrolyte fuel cell is known, in which: an anode gas passage and a cathode gas passage are opposed to each other so as to sandwich an electrolyte membrane-electrode assembly; and an anode gas and a cathode gas respectively flow through the passages in parallel (see PTL 2, for example).
The fuel cell disclosed in PTL 1 is configured such that the flow of the fuel gas and the flow of the oxidizing gas constitute so-called “opposed flow”, and the channels are opposed to each other so as to sandwich the polymer electrolyte layer. With this, respective regions of the gas diffusion layers, the regions each being large in the amount of moisture, are prevented from being opposed to each other via the polymer electrolyte layer. In addition, respective regions of the gas diffusion layers, the regions each being small in the amount of moisture, are prevented from being opposed to each other via the polymer electrolyte layer. As a result, the increase in the local variations in the amount of electric power generation in the electrode can be suppressed.
Moreover, in the polymer electrolyte fuel cell disclosed in PTL 2, the humidity of the anode gas is increased to be higher than that of the cathode gas. With this, in the vicinity of an entrance of the cathode gas passage, moisture diffuses from the anode gas flowing through the vicinity of an entrance of the anode gas passage and moves from an anode electrode side to a cathode electrode side. In contrast, in the vicinity of an exit of the anode gas passage, moisture moves from the cathode electrode side to the anode electrode side. Therefore, supply and discharge control of the moisture in the entire fuel cell can be appropriately performed, and an electric power generating performance of the fuel cell can be maintained successfully.